Discovery of Pulsation Dropout and Turn-on during the High State of the Accreting X-Ray Pulsar LMC X-4

Abstract

Two NuSTAR observations of the luminous X-ray pulsar LMC X-4 in October and November 2015 captured several bright accretion flares from this source, which has a long history of stable pulse and superorbital behavior. We present a timing analysis of these data in which we detect a rapid pulse "turn-on" in association with the accretion flares, during which the source reaches super-Eddington luminosities. Pulsations, which are normally seen from this source, are found to only occur for approximately one hour before and during the bright flares. Beyond one hour before and after the flares, we find pulsations to be weak or nonexistent, with fractional RMS amplitudes of less than 0.05. At the onset of the flare, the pulse profiles exhibit a phase shift of 0.25 cycles that could be associated with a change in the emission geometry. This increase in pulse strength occurring well before the flare cannot be explained by the propeller effect, and potentially offers a connection between the magnetic properties of pulsars that accrete close to their Eddington limits and ULX pulsars.

Discovery of a 2.8 s pulsar in a 2 d orbit High-Mass X-ray Binary powering the Ultraluminous X-ray source ULX-7 in M51

“…This is the first time that such strong changes in pulsation amplitude are observed on timescales as short as hours for a PULX. We note that a similar pulsation drop-out has been recently found in NuSTAR data of LMC X-4 when the source was close to the Eddington luminosity (Brumback et al 2018). During observations B and C, the PF did not show the characteristic increase with energy that was observed in A and in other PULXs, while the B and C X-ray spectra did not show significant variations above a few keV with respect to that of observation A.…”

Discovery of a 2.8 s Pulsar in a 2 Day Orbit High-mass X-Ray Binary Powering the Ultraluminous X-Ray Source ULX-7 in M51

“…This is the first time that such strong changes in pulsation amplitude have been observed on timescales as short as hours for a PULX. We note that a similar pulsation dropout was recently found in NuSTAR data of LMC X-4 when the source was close to the Eddington luminosity (Brumback et al 2018). During observations B and C, the PF did not show the characteristic increase with energy that was observed in A and other PULXs, while the B and C X-ray spectra did not show significant variations above a few keV with respect to that of observation A.…”

Variability in high‐mass X‐ray binaries

“…Most sources show significant pulse‐to‐pulse variability, i.e., each individual pulse might deviate strongly from the average pulse profile. Sometimes this leads to missing single pulses (Göğüş et al ) or the vanishing of detectable pulsations altogether (Brumback et al ). To circumvent the problem of strong pulse‐to‐pulse variability, different authors have used techniques like “pulse‐height resolved spectroscopy” (Klochkov et al ) and “pulse‐to‐pulse” analysis (Fürst et al ), however, at the expense of signal‐to‐noise ratio (S/R) and phase resolution.…”